Does anyone know where one might go to buy the retired vehicles?
Although I suspect since the transition will take a long time, the "retired" vehicles will be kept for spare parts where the old vehicles are still being used, and many will still be kept just in case.
My son had looked into this and it looks like you can't.
Some of it is due to the LLV being manufactured with exemptions from certain US car standards. Some of it is due to a obligation (with Grumman IIRC) that the LLV be destroyed after retirement.
We settled for a 31k mi 1992 Buick with the same Iron Duke engine.
Why do you think that? These are terrible general purpose vehicles that have hundreds of thousands of miles on them already and have atrocious fuel efficiency even at what they are good at doing; start-stop driving.
Continuing to use these for another 50k miles is probably worse for the environment than buying a brand new hybrid or electric vehicle.
Note: USPS is exempt from state regulations because it is a U.S. federal government agency. That is also why they don't have state license plates; they don't need them.